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DEVELOPMENT OF A NONCONTACT PROFILING SYSTEM

Modern profiling systems use a spring-loaded road-following wheel, equipped with a linear potentiometer, to measure the relative displacement between the road surface and a vehicle-borne reference. The requirement for a follower- wheel to remain in firm contact with the road surface restricts the operating speeds and introduces reliability problems that are avoided by noncontacting transducers. A noncontacting displacement transducer was developed which operates on the principle of detecting phase change of a sound wave as a function of path length. This system was evaluated in the laboratory and in the field to ensure that it met the desired specifications for highway profile measurements. Known waveforms with a wavelength of 0.305 m (1.0 ft) were profiled at a vehicle speed of 26.8 m/sec (88 ft/sec). The acoustic probe averages over an area 10.16 x 15.24 cm (4 x 6 in.), which is about the tire contact area for passenger car tires. The averaging process causes the probe to be vertually insensitive to small pebbles, sand and the texture of the pavement surface. /FHWA/